This invention relates to an electrolyte for a rechargeable cell, particularly but not exclusively a lithium cell or a lithium ion cell.
Rechargeable cells are known which incorporate an organic electrolyte with a lithium salt such as lithium hexafluorophosphate, a cathode of an insertion material such as titanium disulphide or lithium cobalt oxide into which lithium ions can be inserted, and an anode either of metallic lithium or a lithium alloy (xe2x80x98a lithium cellxe2x80x99) or of a different insertion material (xe2x80x98a lithium ion cellxe2x80x99) such as graphite. If such a cell is overcharged it may be degraded; this may result from oxidation of the organic solvent, or changes to the electrodes such as deposition of metallic lithium in a lithium ion cell. Overcharging may be prevented by monitoring the voltage of each cell in a battery and regulating it to remain below the voltage threshold at which degradation may occur.
A complementary or alternative approach is to incorporate a redox chemical into the electrolyte which is oxidised at a voltage above the normal cell voltage but below the voltage at which degradation occurs. For example U.S. patent application Ser. No. 744,344 (W. K. Behl) refers to the use of lithium iodide (which is oxidised at about 2.8 V), and describes the use of lithium bromide (which is oxidised at about 3.5 V relative to lithium) as organic electrolyte additives to provide overcharge protection; EP 0 319 182 B (EIC Labs/Abraham et al) describes the use of metallocenes for this purpose, which have redox potentials relative to lithium in the range about 1.7 to 3.7 volts, such as ferrocene for which the redox potential is 3.05 to 3.38 V. However these redox chemicals are not suitable for use in cells where the normal charging cell voltage is above 3.8 volts.
According to the present invention there is provided an electrolyte for use in a rechargeable cell whose voltage exceeds 3.8 volts during charging, the electrolyte including a redox chemical comprising substituted aromatic molecules in which xcex1 protons are either absent, or are stabilised by molecular geometry against loss from the ionised form of the molecule.
The term xcex1 protons refers to protons attached to carbon atoms next to an aromatic ring. For example the aromatic molecules may be a substituted benzene of the general formula C6R1R2R3R4R5R6 in which:
a) R1 to R6 are selected from trihalomethyl groups, C2 to C10 alkyl groups, or halogen-substituted C2 to C10 alkyl groups; or
b) R1 to R5 are as specified in (a), and R6 is H or a halogen.
In a preferred embodiment R1 to R6 are all C2 to C5 alkyl groups, preferably ethyl groups.
The voltage at which such redox chemicals undergo oxidation varies with the nature of the substituents, but can have values between 3.8 and 5.0 volts relative to Li/Li+. The oxidised form, which is a cation, must be sufficiently stable that it can diffuse through the electrolyte to the other electrode where it is reduced back to its original, unionised, form before it undergoes any competing reactions.
The electrolyte will comprise other ingredients, in particular a salt, and an organic liquid or a polymer. For example it might comprise a solution of lithium hexafluorophosphate (1 molar) in a solvent comprising a mixture of organic carbonates such as ethylene carbonate, propylene carbonate, diethylcarbonate and/or dimethylcarbonate. Alternatively it might comprise a polymer consisting principally of vinylidene fluoride, with a plasticizer such as propylene carbonate and a salt such as lithium perchlorate, which form a gel-like solid electrolyte. In either case the redox chemical must be sufficiently soluble in the other organic ingredients, and it must be able to diffuse through the thickness of the electrolyte.
The invention also provides a rechargeable cell including an electrolyte as specified above. In such a cell the active cathode material is typically LiCoO2, LiNiO2, LiMnO2, or LiMn2O4 which operate at least partly in the range above 4.0 V, or mixtures of such oxides, or mixed oxides. Although the active anode material might be metallic lithium, the use of graphitic carbon (which can insert lithium to form LiC6) is preferred, as it avoids the potential hazards of metallic lithium.